1. Field of the Invention
The present invention relates to a part for a rotary machine, and manufacturing method therefor.
Priority is claimed on Japanese Patent Application No. 2008-204180, filed on Aug. 7, 2008, the content of which is incorporated herein by reference.
2. Description of Related Art
Conventionally, for example, in parts, such as a blade used for a rotary machine, such as a steam turbine or a compressor pump, surface treatment is performed in consideration of thermal resistance, erosion resistance, etc. The steam turbine jets steam, which is a working fluid, toward the moving blade of a turbine to drive the blade. Parts of a rotary machine, such as a moving blade (steam turbine blade) or a rotor of this steam turbine, contact steam directly. Additionally, the compressor (compressor pump) which is used in a chemical plant or the like and compresses various fluids receives power from the outside to rotate an impeller and compress a fluid. Even in such a compressor pump, parts of a rotary machine, such as an impeller or a rotor, contact gas directly.
Here, in parts against which water droplets collide at high speed, for example, in the blade of the steam turbine or the impeller of the compressor pump, there is a problem in that erosion abrasion occurs on the surfaces of the parts against which water droplets collide. As a measure against this problem, for example, a technique of performing cladding of stellite on the impeller is known. Additionally, a technique of forming a hard film on the surface of the impeller is known using a physical vapor deposition (PVD) method, such as ion plating.
However, with the technique of stellite cladding as described above, since a base material is melted, there is a problem in that deformation of the base material becomes greater than with other methods. Additionally, with the technique of stellite cladding, hardness is low compared with ceramic coating or the like. Thus, when stellite is not formed as a thick film in the order of several millimeters, there is a problem in that it is difficult to produce the effect of protecting the base material from erosion abrasion.
Meanwhile, as a method of coating a base material with a hard film by the ion plating method to improve erosion-proofness, for example, as shown in FIGS. 13 and 14, a technique of forming a Cr film 101 on a base material 100 made of stainless steel, and coating a TiN film 102 on the surface of the Cr film 101 to form a multilayer is suggested (refer to Japanese Patent Publication No. 8-30264). However, when such a method is used, particulate scaffolding called droplets (refer to reference numeral 110 in FIG. 14) as shown in a microscope picture diagram of FIG. 14 exists in the surface of the coating layer 102. If the size of the droplets is large and the number of the droplets is large, there is a problem in that the erosion abrasion (drain erosion) by the water droplets increases.
A mechanism where droplets are missing from the surface of a hard film of a conventional steam turbine blade will be described below with reference to schematic diagrams of FIGS. 15A to 15C. As shown in FIG. 15A, a drain (water droplets) 250 which forms steam has a diameter of about 100 to 200 μm, and collides against the hard film 210 provided in the surface of a steam turbine blade. In this case, the drain 250, as shown in FIG. 15B, also collides with droplets 220 which exist on the surface of the hard film 210. The droplets 220 may be missing from the hard film 210 due to the collision of the drain 250. As shown in FIG. 15C, portions where the droplets 220 are missing become recesses 230 in the surface of the hard film 210. Cavitation erosion is increased with the recesses 230 as a starting point. In this case, if the density of the droplets 220 is high, there is a problem in that cavitation erosion may be further increased. Furthermore, even when a portion of a missing droplet 220 remains inside a recess 230, a problem that the same cavitation erosion is increased occurs.
Additionally, forming a film on the surface of a cutting tool by a sputtering method is suggested in the field of cutting tools, such as a ball end mill. Specifically, it is suggested that micro-particles (droplets), which exist on the surface of a cutting tool, having a maximum diameter of 10 μm or less, and an occupying area ratio of 10% or less on the surface, form a film (refer to Japanese Patent Unexamined Publication No. 2006-116633). However, even if the film described in Japanese Patent Unexamined Publication No. 2006-116633 is applied to parts for a rotary machine, such as a steam turbine blade, there is a problem in that the droplets are easily missing. This is because, when droplets having a maximum diameter of 10 μm compared to water droplets (steam) whose diameter is 100 to 200 μm exist on the surface of a film, droplets are easily missing due to the stress concentration at the boundary of the film. Additionally, the area occupied by the droplets in this film surface is relatively large. Accordingly, when droplets are missing and drain erosion occurs, there is a concern that drain erosion may be increased by the interaction between droplet missing spots.
Additionally, a technique of removing droplets formed in a film surface by methods, such as polishing, in advance is also suggested (refer to Japanese Patent Unexamined Publication No. 2005-1088). However, a spot where a droplet is removed in the film surface degrades erosion-proofness. Therefore, with this technique, there is a problem in that erosion is increased with this position as a starting point.
Generally, in order to reduce the particle diameter of droplets and to decrease the number of the droplets, it is necessary to reduce the deposition rate of a film and also to reduce film thickness. However, in order to improve drain erosion-proofness by the hard film, a film thickness of about several micrometers to tens of micrometers is required. When the hard film is formed thickly in this way, the particle diameter and number of droplets also increase according to a thickness. Accordingly, it was difficult to apply a hard film to be used for general cutting tools or the like to, for example, parts for a rotary machine, such as a steam turbine blade, as a drain erosion-proofness coating.
The invention has been made in view of the above problems, and an object thereof is to provide a part for a rotary machine and its manufacturing method, which are excellent in drain erosion-proofness under an environment in which the part for a rotary machine contacts steam directly.